For amateur radio use I found them to be a bit quiet and require me to turn up the AF Gain on my K2 Elecraft rig to near maximum volume before I could hear them.
I figured it would be a great project to try to build an audio booster amplifier from scratch.
The design in this blog post did not meet the objective.
The headset has an impedance of 150ohms. I determined through listening to a 1KHz tone that a peak-peak voltage of 1V maximum was required to drive the headset to a loud volume. However output distortion occurs when greater than 250mV. This distortion could be easily heard in the headphones. The reason has not been determined. Next I will try raising the emitter voltage to greater than 1V.
As a part of the design process a 150Ohm resistor was used. The results were the same when the headphones were actually connected to the output.
Design Input requirements:
- Rail voltage = between 7V and 12V to allow use of a 9V internal battery and an external 12V supply.
- Total current drain = <3mA.
- Frequency range = 200Hz - 3000Hz.
- Maximum input signal amplitude = 100mVp-p.
- Output signal amplitude = 1V.
- Load impedance = 150 Ohms.
The common emitter design is based on achieving a gain of 10.
Calculated and installed component values are shown on the included schematic.
Calculated and measured parameters are shown on the included schematic.
The 3dB filter roll-off points were measured as 200Hz and 1800Hz (approximately).
|250mV Output for 22mV Input. Gain =11.4. No Distortion|
|800mV Output for 100mV Input. Voltage gain = 8. Distorted.|